Polypropylene is typically not suitable for use in producing various films. The physical properties of homopolymers of propylene formed by typical Ziegler-Natta polymerization are highly dependent on the stereoregularity of the polymer itself. Highly stereoregular polymers are generally crystalline, provide desirable high flexural moduli and are formed with a suitable choice of electron donor. These highly crystalline polymers also display high melting points, but innately exhibit low melt flow rates (MFR) that render them generally unsuitable for applications that require high processing rates, such as in injection molding, oriented films and thermobond fibers. Further, conventional polypropylene homopolymer and copolymer products formed from highly crystalline polypropylenes lack sufficient impact resistance for many uses. Polypropylene films are also subject to gelling issues, rendering them unsuitable for many film applications.
The polypropylene homopolymer or impact copolymer resins made from the traditional Ziegler-Natta catalyst based on phthalate or other aromatic containing internal electron donor systems, and a silane or diethers external electron donor, result in a molecular weight distribution (MWD) in the range of 3 to 4.5 and as such, have very low melt strength with no evidence of strain hardening under elongational extension in the melt. The resins with molecular weight distribution in the range of 3 to 4.5 are not suitable in converting processes such as blown film applications either in mono-layer or multi-layer applications due to poor melt strength. Similar behavior is observed in sheeting, deep-drawn thermoforming, and foaming applications. Other converting applications requiring good melt strength for which such polymers are not suitable include profile extrusion, base stock for thermoplastic vulcanizates (TPV), bi-axially oriented polypropylene (BOPP) film, blow molding applications, and the like.
There is a need in the art for polypropylene resins having one or more properties such as improved melt strength, improved stiffness, and the like. There is also a need for such polypropylene resins suitable for use in blown film, in multi-layer applications as replacement for HDPE, sheeting, thermoforming in shallow drawn and deep drawn applications, and/or foaming applications.
Related references include EP 0 757 069 A1; EP 2 000 504 A1; US 2003-088022; US 2007-054997; US 2008-311368; U.S. Pat. Nos. 6,602,598; 5,907,942; US 2003-118853; U.S. Pat. No. 5,752,362; WO 2007/130277; WO 98/44011; and WO 2010/034461.